Tendon transfer surgeries to restore hand function after cervical spinal cord injury are not always as successful as expected. This study will attempt to develop better biomechanical models for predicting the outcome of the brachioradialis-to-flexor-pollicis-longus (Br-FPL) tendon transfer, a procedure that restores lateral pinch following tetraplegia. The PIs hypothesize that the effectiveness of the transferred Br is impaired because of post-operative muscle adaptations and weakness of elbow and wrist extensors. The goal of this study is to characterize the structural and functional changes that take place in transferred muscle, and to more accurately model the impairments associated with the surgically altered tetraplegic limb. Aim 1 will define the maximum potential of the Br-FPL transfer to produce lateral pinch force. A nominal model of the transfer will characterize the clinical ideal, in which the transferred muscle has normal strength and can be maximally activated for its new function. Previous work suggests that this model will substantially overestimate actual pinch forces measured in subjects who have had transfers. Aim 2 will determine the extent to which pre-operative weakness limits force production. Personalized simulations of Br-FPL tendon transfers will be developed for 8-10 surgical candidates based on preoperative assessments of Br cross-sectional area, obtained by medical imaging, and elbow and wrist extension strength. These models should predict pinch force better than the nominal model. Aim 3 will identify post-operative changes to muscle and fiber architecture. The transferred Br will be reimaged and a novel electrophysiological technique will be applied. It is expected that the transferred muscle will have reduced cross-sectional area and lack distal bands of muscle fibers, and that models incorporating these differences will lead to even better predictions of actual outcomes. Aim 4 will test the ability to fully activate the transferred Br during lateral pinch. EMG during maximum voluntary effort for pinch is expected to be less than EMG during maximum resisted elbow flexion (Br's original function), in part due to lack of wrist and elbow extension strength needed for joint stabilization. This work will identify and characterize factors responsible for disappointing surgical results, and this better understanding will lead to better treatment decisions.